: The mass moments of inertia about the X, Y, and Z axes. RZ (rotational mass) is particularly vital for evaluating the torsional response of the building.
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ETABS reports the coordinates (X, Y, Z) of the center of mass per story. This is essential for calculating accidental eccentricity and torsional irregularity (per ASCE 7 Section 12.8.4.2). etabs mass summary by story
Loads like lateral wind or soil pressure are forces , not masses . The summary will only include loads that have a vertical component and are assigned to the mass source. Review your diaphragm assignments to ensure mass is correctly lumped at the center of rigidity.
If ETABS reports total mass = 100,000 N·s²/m, then $W = 100,000 \times 9.81 = 981,000 N = 981 kN$. This must match the sum of dead load plus applicable live load per ASCE 7. : The mass moments of inertia about the X, Y, and Z axes
These coordinates tell you where the "weight" of the floor is centered. If these are far from the geometric center, you’ll have high accidental torsion.
By mastering the Mass Summary by Story table, you ensure your dynamic structural analysis stands on a structurally sound, perfectly weighted foundation. This link or copies made by others cannot be deleted
Multiply the total square footage of your building by an estimated average weight per square foot (e.g., 100–150 psf for concrete buildings, 50–80 psf for steel buildings). If the cumulative mass in ETABS differs from your hand calculation by more than 5% to 10%, re-examine your material properties and load assignments.
The ETABS output table contains several key columns. Here is what they mean:
For Time History analysis, the sum of inertial forces at any time step should equal the base shear. This only holds true if the mass per story is correctly defined. The Mass Summary serves as the baseline for debugging dynamic responses.
: Confirm that the mass decreases or changes logically from the roof down to the base.